1. Field of the Invention
The present invention relates to an apparatus for detecting deterioration of a catalyst disposed in a exhaust system of an engine and acting to purify an exhaust gas.
2. Related Art Statement
Hitherto, there has been a system having oxygen sensors in the upper and the lower streams of a catalyst. A system of the type described above compensates, by means of an output signal from a lower stream oxygen sensor thereof, scattering of output signals of the upper stream oxygen sensor due to, for example, a change with the passage of time. The output signal from the lower stream oxygen sensor is, due to the characteristics thereof, changed in a period longer than that of the output signal from the upper stream oxygen sensor because of the storage effect of the catalyst.
However, the storage effect degrades if the catalyst deteriorates and unburnt gases such as HC, CO, NOx gases are introduced into the lower stream of the catalyst, causing the period of the output signal from the lower stream oxygen sensor to be shortened (the delay time to be shortened) to a period which is substantially the same as the period of the output signal from the upper stream oxygen sensor. Furthermore, the amplitude of the output signal from the lower stream oxygen sensor becomes enlarged in comparison to that realized when the catalyst is in the normal state.
Accordingly, an apparatus for detecting deterioration of a catalyst has been known which is adapted to a method arranged in such a manner that the period ratio {(the period of the upper stream sensor)/(the period of the lower stream sensor)} the output signals from the upper and the lower stream oxygen sensors is obtained so as to discriminate that the catalyst has deteriorated if the aforesaid period ratio becomes smaller than a predetermined value or if the amplitude of the output signal from the lower stream oxygen sensor has exceeded a reference value (refer to, for example, U.S. Pat. No. 4,739,614).
Another method has been disclosed in Japanese Patent Laid-Open No. 2-207159 in which the discrimination of the deterioration of the catalyst is commenced if conditions for discriminating deterioration of a catalyst are satisfied, that is, under the conditions that warming up of the engine has been sufficiently performed and the catalyst has been satisfactorily warmed because the temperature of water for cooling the internal combustion engine is included by a predetermined range, the load is included by a predetermined range, a predetermined time has passed after the engine had been started, and the idle switch is switched on. In this method, the deterioration of the catalyst is detected in such a manner that the air/fuel ratio of the internal combustion engine is forcibly switched from a rich state to a lean state, and a discrimination is made that the catalyst has deteriorated if the time taken from the moment the air/fuel ratio has been forcibly switched from the rich state to the lean state to a moment the output from the lower stream oxygen sensor has been switched from the rich state to the lean state is shorter than a predetermined time.
However, the aforesaid conventional method has a problem that the detection of the deterioration of the catalyst cannot be performed accurately because th catalyst deterioration discriminating process is performed although the air/fuel ratio of the internal combustion engine is not converged to the neighborhood of a theoretical air/fuel ratio due to its basic structure in which the catalyst deterioration discriminating process is performed if the engine operation state, that warming up of the engine has been performed sufficiently and that the catalyst has been warmed up satisfactorily, is satisfied.